Comparison of genetic parameters of weight traits estimated using cross sectional and longitudinal analyses of animal models with two types of contemporary groups in spanish Merino lambs

Genetic parameters and breeding values for weights at weaning (45 days - W45) and slaughtering (75 days - W75) were estimated with 70,635 records of weights recorded from 17,993 lambs of Spanish Merino sheep descendants from 402 sires and 9614 dams. Cross sectional (CS) and longitudinal (random regression - RR) analyses of animal models were used to get the estimates. Contemporary groups included as fixed factors in these models were based either on grouping animals in herds (H) or in clusters of herds with similar environmental levels (EL) defined on the bases of environmental descriptors. Cross validation methods were used to fit models to 80.0% observations randomly chosen and to predict phenotypic values with these models and the remaining 20.0% of the observations. Correlations between observed and predicted values obtained with CS analysis with EL were an average of 4.4% and 4.9% higher than those models with H for W45 and W75, respectively. Estimates of direct heritability and maternal heritability for W45 obtained with CS analysis increased from 0.13 to 0.15 and from 0.54 to 0.58, respectively, when using EL instead of H contemporary groups. The genetic covariance between direct and maternal effects changed from −0.49 to −0.55. Direct heritability of W75 also increased from 0.25 to 0.35. The estimations of heritability obtained with RR analysis showed similar patterns. When using EL contemporary groups direct heritability of W45 and W75 increased 12.7% and 10.7%, respectively, and reliabilities of the estimated breeding values (EBV) for W45 and W75 increased 32.7% and 34.7%, respectively. Increments observed when EBVs were estimated with RR were only 10.8% and 3.5% for W45 and W75, respectively. Expected genetic gains to be obtained in the selection program of the Spanish Merino breed may expectedly increase up to almost 40.0% with this method of clustering herds to constitute new contemporary groups.